ENDOPHYTE-HOST ASSOCIATIONS IN GRASSES. XXV. THE GRASS/FUNGUS INTERFACE IN EPICHLOE AMARILLANS

Authors: WHITE, JAMES - PLANT PATH., RUTGERS UNIV; Bacon, Charles; Hinton, Dorothy

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: There are several kinds of fungi that grow within grasses and the grasses show no adverse effect from this added fungal component. The fungi are referred to as endophytes. Since the fungi are grown within grasses and cannot make their own food, all nutrients they receive must originate from the grass. The fungus makes several chemicals from the grass derived nutrients, several of which are mammalian toxins. It is known that endophyte-infected grasses grown under high nitrogen produce more toxins. It is not known how this is achieved at the fungus level since the points of contact between the fungus and grass are not typical of other fungi that forcibly obtain nutrients from plants. Scientists at RRC collaborated with scientists at Rutgers University to study the zone through which nutrients must pass between the fungus and the grass, referred to as the interface. The interface is the area where fungus and grass cells meet. This study was performed using the electron microscope to observe the ultrastructural nature of the interface. No obvious alterations indicative of a disease or death were observed at the interface However, the grass cells were thicker at the interface indicating that they are being altered, possibly at the level of cell membrane. An untested but logical mechanism of nutrient flow from grass cells to fungus cells is presented to explain how nutrients flow between these two organisms.

Technical Abstract: An ultrastructural examination of stromata of Epichloe amarillans is conducted to assess changes in host tissues associated with development of stromata on grasses. Notable changes are observed in the epidermal layers of leaves and inflorescence primorida embedded within stromata. Epidermal cells within stromata show progressive disintegration of the cutical and wall layers. These alterations effectively break down the epidermal barrier to the flow of nutrients and water to fungal tissues that develop outside the plant tissues. Changes are also observed in the degree to which plant tissues show storage of carbohydrate as starch. Plant tissues without stromata showed abundant starch grain formation, while tissue embedded within stromata were not seen to develop starch grains. Mycelium was observed to associate closely with host cells. No penetration of cells of the mesophyll, phloem, or xylem were observed, however, some alteration of the host cell walls was detected. Host cell walls proximal to hyphae were often seen to be slightly thicker than walls adjacent to other plant cells. This is suggested to be evidence that the plant is responding defensively to parasitism by the fungus. Unseen atlerations to host cell membranes is hypothesized. A mechanism of nutrient flow from host to stromal mycelium is also discussed.